Hi T.Jouko,
I have just tried to set the megainc template as default and it looks fine here I think. Please check the screencast in the hidden content box and share a screenshot of where you have that error.
Moreover, I have seen that you are using helix3 and helix ultimate both framework at the same time in your site. Please note that, since they are different in many areas, so conflict may happen and should not be used together. Kindlt, remove one and keep one. I would prefer to keep the Helix Ultimate framework as this is the latest one from us.
https://share.cleanshot.com/7Y9hWQrr
Best regards,